This invention pertains to a transmission system, and a station unit and remote units to be used in the transmission system, and more particularly to a transmission system, wherein plural remote units are connected to a station unit in star form, and in regard to the transmission system communicating between the station unit and the remote units by using fixed length cells, specifically, communication in the direction from the remote units to the station unit (henceforth, described as upward) being done in TDMA (Time Division Multiple Access) system, while communication in the direction from the station unit to the remote units (henceforth, described as downward) being done in TDM (Time Division Multiplex) system.
One example of a conventional transmission systems is shown in FIG. 1. Also, a structure of a general station unit in this transmission system is shown in FIG. 2, and that of a remote unit in FIG. 3.
This optical transmission system, between one station unit 1 and n remote units 21xcx9c2n with different transmission length, carries out one-core two-way communication by using different wave bands. In other words, broadcasting-type communication with fixed length cells is done in the downward direction, while TDMA system communication with fixed length cells is used in the upward direction.
Since in an optical transmission system of this type, optical signals are branched and linked with a coupler 3, it is necessary to include delay control so that upward cells from each remote unit, 21xcx9c2n, may not collide. Thus, when each remote unit, 21xcx9c2n, starts up, one carries out distance measurement control for measuring the transmission distance by the command of the station unit 1, and sets a delay for each remote unit, 21xcx9c2n. 
Allocation of upward bands to each remote unit, 21xcx9c2n, is always commanded by the station unit 1 to each remote unit, 21xcx9c2n. In an upward time slot, an area for distance measurement is allocated appropriately, and the station unit 1 receives monitoring control cells transmitted by a certain remote unit in this area, and the bands are not allocated to other remote units. Each remote unit, 21xcx9c2n, is, however, either in the state of power-off or power-on. In the power-off state, monitoring of the transmission line between the remote unit and the station unit 1 and the state of the remote unit itself are not possible. Thus, when it switches from a power-off state to a power-on state, after distance control by the station unit 1, it switches to an operation state where communication is possible, transmitting and receiving user cells having user information, and also monitoring control cells for monitoring the transmission line between the remote unit and the station unit 1 and the remote unit itself. When the user does not use the band, it transmits idle cells upward for adjusting the rate.
However, in the conventional transmission system, even when a user does not use upward bands, there is no means for informing the station unit 1 of it, and it transmits idle cells to the allocated bands, and in order to monitor the transmission line between the station unit 1 and the remote unit or the state of the remote unit itself, the station unit and the remote units 2n need to be in a normal communication state, resulting in a problem of inefficient utilization of the upward bands.
Once the power of a remote unit is turned off, it needs distance measurement control after the power is turned on again. Thus even if one tries to save power consumption without user communication, it causes a problem that the restart-up of the remote unit takes time, once its power is turned off.
Thus, an object of this invention is to provide a transmission system, a station unit and remote units, which can monitor the transmission lines and the remote units under conditions of controlled power consumption.
Another, other object of this invention is to provide a transmission system, a station unit, and remote units, which intend to utilize effectively the bands for user cells.
Further object of this invention is to provide a transmission system, a station unit, and remote units, which shorten the time for restarting user cell communication.
According to the first feature of the invention, a transmission system comprises plural remote units connected with a station unit in star form, each communicating with fixed length cells in the upward direction from the remote units to the station unit by TDMA system, and in the downward direction from the station unit to the remote units by TDM system, wherein when the station unit detects a remote unit in the standby state so as to save power consumption, it allocates the bands allocated to the remote unit in the standby state to a remote unit in a normal state, and transmits and receives monitoring control information periodically to and from the remote unit in a standby state so as to monitor the transmission line and the state of the remote unit, allocating again the bands according to its result.
According to the second feature of the invention, a transmission system comprises plural remote units connected with a station unit in star form, each communicating with fixed length cells in the upward direction from the remote units to the station unit in TDMA system, and in the downward direction from the station unit to the remote units by TDM system, wherein a remote unit turns into a standby state, it transmits and receives a monitoring control cells periodically to and from the station unit so as to monitor the transmission line and the remote unit itself, and the station unit controls so that the bands allocated to the remote unit may be allocated to a remote unit in a normal state excluding the bands for transmitting upward monitoring control cells when the monitoring control cells indicate the standby state, and the remote unit concerned may be returned to the band allocation in a normal state, when the remote unit is released from the standby state.
According to the third feature of the invention, a station unit, in a transmission system comprises plural remote units connected with the station unit in star form, each communicating with fixed length cells in the upward direction from the remote units to the station unit by TDMA system, and in the downward direction from the station unit to the remote units by TDM system, comprises means for separating the upward signal input from a remote unit and the downward signal output to a remote unit, means for receiving the upward signal thus separated means for dividing the received upward signal into user cells and monitoring control cells, means for controlling band allocation according to the band allocation command signal and outputting the band allocation information, means for processing monitoring control cell wherein upward monitoring control cells from the dividing means are input to execute terminal processing for collecting a transmission-line warning or a remote-unit warning, and also, if a power-off or a standby of a remote unit is indicated by the monitoring control cells, the band allocation command signal is output to get the band allocation information, means for processing termination of user cells input from the dividing means and generation of user cells, means for multiplexing user cells from the user-cell processing means and monitoring control cells from the monitoring control cell processing means, and means for transmitting an output of the multiplexing means via the dividing means.
According to the fourth feature of the invention, a remote unit, in a transmission system comprises plural remote units connected with a station unit in star form, each communicating with fixed length cells in the upward direction from the remote units to the station unit by TDMA system, and in the downward direction from the station unit to the remote units by TDM system, comprises a wavelength separating and multiplexing means for separating the downward signal input from the station unit and the upward signal output to the station unit, an downward signal reception means for receiving the downward signal after the signal separating, a cell division means for dividing the received signal concerned into user cells and monitoring control cells, a power-off monitoring means for detecting a power-off state of the remote unit, a standby state monitoring means for detecting the remote unit being in the state of power-off, a monitoring control cell processing means wherein downward monitoring control cells are input from the cell division means for getting band allocation information to output a cell transmission command signal, and also a power-off indication signal is input from the power-off monitoring means and a standby state indication signal from the standby state monitoring means to generate upward monitoring control cells, a user cell processing means for terminal processing of user cells input from the cell division means and generation processing of user cells, a cell multiplex means for executing cell multiplex processing of user cells from the user cell processing means and monitoring control cells from the monitoring control cell processing means according to a cell transmission command signal from the monitoring control cell means, and an upward signal transmission means for transmitting an output of the cell multiplex means concerned via the cell division means.